The present invention relates to a solid wire used for arc welding, and particularly relates to a solid wire for carbon steel usable for arc welding of a sheet.
Recently, improvement in fuel consumption of a car is increasingly demanded in the light of the environmental issue. To response to this, change of a steel sheet to be used is progressively tried from usual mild steel having a tensile strength of 300 MPa or less to a high strength steel sheet having a tensile strength of 400 MPa or more so that sheet thickness is reduced to achieve lightweight.
Here, when sheet thickness is reduced, while substantially no difficulty is found in spot welding, a phenomenon that a steel sheet is locally melted due to arc heat, and eventually a hole is opened, so-called burn through easily occurs in arc welding.
Increase in strength of a steel sheet is typically achieved by cooling control during rolling, and increasing the amount of an element to be added into the steel sheet. However, when the amount of the element to be added into the steel sheet is increased, viscosity of a molten pool during arc welding is increased, thereby blendability is degraded and a weld toe is lose in thickness, so-called undercut tends to occur.
Furthermore, in a point of welding procedure, when welding speed is increased to improve efficiency of welding, arc precedes the molten pool, and thus arc tends to directly impinge upon a molten surface, so that burn through easily occurs. Even if burn through is not brought, when melting reaches to a back of the sheet material (this is sometimes called uranami welding), hot crack is induced extremely easily.
For improving burn-through resistance, JP-A-2001-96392 describes that heat input per deposition is decreased by using a wire having a small diameter of 0.9 mm or less, which is specified in electric resistivity, so that burn through hardly occurs (that is, burn-through resistance is improved).
Moreover, specification of Japanese Patent No. 2922814 describes that Si+Mn and Si*(Si+Mn) in a composition of a wire are specified, and furthermore Ar is mixed with an extremely slight amount (3 to 7%) of O2 gas, thereby burn-through resistance is improved. Such a technique is based on a synergistic effect of an effect of decreasing heat input per deposition by appropriately increasing electric resistivity of the wire, and an effect of decreasing depth of penetration by increasing a ratio of Ar in shielding gas.
Moreover, JP-A-9-94667 and specification of Japanese Patent No. 3345883 describe use of a special welding power supply tip applied with ceramic at an end. This increases electric resistance heat generation between a current supply point at an end of the welding power supply tip and an arc generation point, thereby heat input per deposition is decreased, so that burn-through resistance is improved, and occurrence of undercut is suppressed since arc force is restrained due to current reduction (that is, undercut resistance is improved).
Furthermore, JP-A-2005-254284 describes that Ar and CO2 are used for shielding gas, and a large amount of O2 (5.5 to 15%) is mixed to the shielding gas to change a convection direction of a molten pool, thereby undercut resistance is improved.
While it is previously experientially known that the burn-through resistance and the undercut resistance can be improved by vertical, downward welding, there is a difficulty that a welding position is limited, in addition, when a downward angle is excessively large, welding is hard to be controlled since drop of a bead may occur, or a large amount of spatter may be generated.
However, the wire having a small diameter as described in JP-A-2001-96392 tends to buckle due to the small diameter, and has a difficulty that weldability is degraded since stability of wire feed is bad. Moreover, there is a difficulty that such a wire small in diameter causes increase in cost, leading to increase in welding cost.
Moreover, the difficulty of bad burn-through resistance cannot be overcome only by using a simple wire as described in specification of Japanese Patent No. 2922814. Furthermore, there is a difficulty that when the ratio of Ar is increased in shielding gas in such a way, cost of the shielding gas is increased, leading to increase in welding cost.
On the other hand, use of the special welding power supply tip as described in JP-A-9-94667 and specification of Japanese Patent No. 3345883 increases tip cost, leading to increase in welding cost.
In the arc welding method described in JP-A-2005-254284, there is a difficulty that since the special shielding gas is used, cost of the shielding gas is increased, leading to increase in welding cost. In addition, since a large amount of oxygen is mixed, a large amount of slag or spatter is generated. The slag or spatter leads to degradation in painting adaptability of a weld. Furthermore, there is a difficulty that as the amount of oxygen contained in weld metal is significantly increased, inclusions are increased, consequently hot crack tends to occur (that is, crack resistance is bad).
In addition to the difficulties, for versatile and general sheet welding, it is required for the wire that hardness of weld metal is equal to or higher than hardness of base metal, brittle fracture does not occur in the weld metal, and stability of wire feed is good for smoothly performing welding.